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Preprocessor directive: #elif not defined?

Is there a preprocessor directive that checks if a constant is not defined. I know the directive #ifndef

, but I am also looking for the directive #elif not defined

. Does it exist #elif not defined

?

This is how I will use it:

#define REGISTER_CUSTOM_CALLBACK_FUNCTION(callbackFunctName) \
    #ifndef CUSTOM_CALLBACK_1 \
        #define CUSTOM_CALLBACK_1 \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #elif not defined CUSTOM_CALLBACK_2 \
        #define CUSTOM_CALLBACK_2  \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #elif not not defined CUSTOM_CALLBACK_3 \
        #define CUSTOM_CALLBACK_3  \
        FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) \
    #endif
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1 answer


What about

#elif !defined(...)

But you're in big trouble - the final one \

rules out other directives - or rather, makes them illegal. This way, even with valid syntax, your definitions won't do what you want.



You will need to wrap the initial definition inside the conditions.

#ifndef CUSTOM_CALLBACK_1
    #define CUSTOM_CALLBACK_1 
    #define REGISTER_CUSTOM_CALLBACK_FUNCTION(callbackFunctName) \
    FORWARD_DECLARE_CALLBACK_FUNCTION(callbackFunctName) 
#elif !defined(CUSTOM_CALLBACK_2)
    //.....
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